Abstract: A sub-frequency measurement method for weak signals with unknown frequency was presented. Due to the sensitivity of the state transitions of the phase trajectories of the coupled Duffing system to the weak signal input, the detection for a weak signal submerged in strong noises was implemented to solve the difficult problems of weak signal detection. Based on the proposed method, the problem on misjudgment of weak signal detection method based on single Duffing oscillator under the influence of noise effectively was solved. And the limitation had been broken that the existing detection methods of weak signal based on chaotic oscillator could only detect the weak signal with known frequency. Simulation results showed that the method could indeed detect the frequency of weak periodic input signal more accurately, resulting in the improvement of the weak signal detection techniques.